Bulletin of the National Speleological Society - ISSN 0146-9517Volume 27 Number 4:109-132 - October 1965

A publication of the National Speleological Society

The Origin of Limestone
Caverns: A Model from the Central Mendip Hills, England Derek C. Ford

Abstract

Theories of limestone cavern
genesis are divided into three conflicting groups: vadose, water table, and
deep phreatic. Type examples for all three can be found, at short distances
apart and occupying similar positions in the regional geologic structure, in
caverns of the central Mendip Hills in southwest England. Representative caves
are described:

1. St. Cuthbert's Swallet
is a drained phreatic system evidencing well integrated cave development to
a depth not less than 280 feet beneath the contemporary water table.

2. Swildon's Hole is a water
table system with four major cave levels. Accordance of the elevations of principal
passages with defined water tables becomes progressively closer through the
lower (younger), levels.

3. G.B. Cave is a largely
vadose system, descending through the same range of absolute elevation as the
phreatic and water table caves. At its lowest points, passages assume a water
table form and are abruptly reduced in dimensions.

4. The Cheddar Caves and
Wookey Hole discharge the flow of the above-mentioned systems. They are composed
of both deep phreatic and water table cave elements.

The evidence is reconciled
in a single model: in the vertical plane, cave passages make deep loops below,
and returning to, a linear water table which they have created. The amplitude
of this phreatic loop penetration becomes reduced with growth in the quantity
of minute groundwater conduits which may be utilized to guide principal passages.
St. Cuthbert's Swallet and Wookey Hole are parts of single phreatic loops. Swildon's
Hole and the Cheddar Caves contain several loops, with adjustments to negative
shifts of an allogenic base level. G.B. Cave is the youngest in the sample and
occupies an uppermost zone of the rock which was drained and air-filled before
a stablized water table existed locally.

The three largest systems
are of substantially different ages. The time at which development commenced
appears to have been determined by an interplay of three geomorphic factors:
volume of available groundwater, hydraulic gradient in the rock and the efficiency
of overground discharge of run-off.